The amounts of harmful substances contained in drinking water and wastewater are strictly regulated, and there is a need for techniques to measure harmful trace metals and other harmful substances in solution with maximum accuracy.
Of the harmful substances, arsenic (As) in particular is an element that easily enters the human body via drinking water and food, and is extremely harmful to humans because it accumulates in the body, causing arsenic poisoning and even death.
For example, arsenic from minerals may seep into ground water, and be ingested by humans as drinking water. Moreover, arsenic may be contained in the wastewater from arsenic mines, where arsenic-containing minerals are mined and used to manufacture arsenous anhydride, and from zinc smelters, where arsine (AsH3) is produced in the cadmium removal purification process when reducing and collecting cadmium. Waste material containing arsenic compounds is also produced by semiconductor plants, in which gallium arsenide (GaAs) and iridium arsenide (IrAs) are used, and arsenous anhydride is also sometimes used as a clarifying agent in the process of manufacturing specialty glass such as optical class, electric glass and the like. At certain times, arsenic compounds are also used as wood preservatives and termite poisons.
The effect on society is severe when such arsenic and arsenic compounds contaminate household water. There is a need for methods whereby arsenic and arsenic compound concentration can be easily and accurately detected not only in drinking water but also in all household water. A spectrophotometric method such as a molybdenum blue method is known as a method for analyzing metal (electrochemical substances) in a solution.
The molybdenum blue method is capable of analyzing arsenic and the like by an easy process that does not require expensive equipment, but it cannot easily detect trace quantities.
Trace substances can be accurately detected by the HPLC-ICP/MS method, which combines high performance liquid chromatography (HPLC) with inductively coupled plasma mass spectroscopy (ICP/MS), but this is a specialized method that can only be performed at certain facilities and requires complicated operations.
By contrast, electrochemical measurement allows trace substances to be detected with easy operations, and also allows quantitative measurement.
One method of electrochemical measurement is the method commonly called “stripping voltammetry”.
An outline of this method is explained based on the system illustrated in FIG. 1. An electrolyte solution la containing a substance to be measured (such as metal ions) is placed in measurement container 1, which is sealed with seal member 1b. Working electrode 2 and counter-electrode 3 are immersed at a fixed distance from each other in electrolyte solution 1a in measurement container 1. Reference electrode (standard electrode) 5, which is a saturated calomel electrode or the like, is electrically connected to working electrode 2 via capillary 5a, and is also connected to potentiostat 4 via wiring 5b. Potential sweeper 4a and recorder 4b are also connected to potentiostat 4.
When a metal is analyzed in solution, the potential of working electrode 2 is swept from natural electrode potential toward negative potential by potentiostat 4 so that the metal ions in electrolyte solution 1a are electrodeposited one by one on the surface of working electrode 2 to thereby form an electrodeposit (reductive concentration). The potential of the aforementioned working electrode 2 is then swept in the positive direction by means of potentiostat 4 to thereby oxidize and elute the electrodeposit in electrolyte solution 1a. Because each of the metals in the electrodeposit is eluted at a specific oxidation potential, the metal components in electrolyte solution 1a can be analyzed by detecting current changes in response to potential changes at working electrode 2 (peak current relative to potential) when each of the metals is eluted in electrolyte solution 1a (anodestripping).
Patent Document 1 discloses an analysis method using stripping voltammetry, and discloses using a mercury electrode, carbon electrode, mercury-modified carbon electrode, gold-amalgam electrode or the like as the working electrode in this method.
However, accurate analysis of trace quantities has not been achieved with these electrodes due to such problems as low potential windows, heavy background current and the like.
Patent Document 1: Japanese Patent Application Laid-open No. H6-27081